Method of producing a cooled electronic assembly

ABSTRACT

Cooling equipment for use in electronic systems of type having a substrate with a plurality of heat-generating electronic components mounted thereon is disclosed. A hat assembly is provided having a plurality of cavities in which pistons can be slidably mounted. The hat assembly is placed adjacent to the substrate such that the pistons are next to the components. The pistons are then adjusted and fixed in the hat assembly so that a desired gap is maintained between the pistons and the components. A cooling plate is attached to the assembly.

This is a division of application Ser. No. 764,278, filed Aug. 9, 1985,and now U.S. Pat. No. 4,628,990.

BACKGROUND OF THE INVENTION

The present invention relates to cooling equipment for an integratedcircuit chip.

For integrated circuits (IC) mounted on a print wired board or a ceramicsubstrate, air cooling equipment is usually used. In such equipment, thecooling air is blown against the heat sink or the case. However, for thefollowing reasons, use of air as a cooling medium to carry the heat froman IC has some disadvantages and problems.

The first reason is the necessity for relatively large air conditioningcapability to lower the air temperature because the temperaturedifference between the heat sink or IC case and air is large. A largeamount of air must be sent into the unit to remove the heat generated bythe unit, since the heat capacity of the air is small. Further, moderndata processors comprise high density and high power ICs to speed upprocessing and signal transmission, thus increasing both the quantity ofheat and the density of heat generated. Accordingly, the air conditionerneeds high capacity and, inside the unit, an air blower must give highairflow and exhausting ability to compensate for the reduction of theair path inside the unit caused by high density mounting of componentsand to exhaust the great amount of heat from the unit. The necessity foran air conditioner with high cooling capacity and large throughput alsocan cause excessive noise. Further, since it is difficult for the airstream to be distributed equally in the unit, the temperature of the ICsmay rise locally, causing deterioration of the reliability of the unit.

To solve these defects, an example of liquid cooling unit is proposed inthe article entitled "A Conduction-Cooled Module for High-Perfomance LSIDevices", by S. Oktay et al, published in the IBM J. Res. Develop., Vol.26, No. 1, January 1982, pages 55-66. Referring to FIG. 1(b) of thearticle, the surface of a chip mounted on the substrate is contacted byspring loaded piston so that the heat from the chip is conducted to a"hat" via the piston and helium. The hat is cooled by cooling mediumutilizing a water cooled cold plate heat sink.

The module as proposed by Oktay et al has the following shortcomings.First, since the semiconductor chip mechanically contacts the piston, itis impossible to achieve electrical isolation between the chip and thecooling medium. As the cooling medium is electrically connected to theearth terminal of a compressor or a pump for sending out the coolingmedium, if a conductive cooling medium is used, the chip can misoperate.Next, a vibrating system expressed in the equation of W_(o) =√k/m (W_(o)shows sympathetic angular frequency, K shows constant of the spring, andm shows mass of the piston) is made up by the piston and the spring. Ifthe power including a sympathetic frequency element from outside putspressure on the module, vibration of the piston can destroy the chip.Finally, creep transformation can occur because the solder forconnecting the substrate is always under mechanical pressure. Thistransformation is particularly significant in the case of using solderwith a low melting point.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide coolingequipment and a method for assembly thereof, which give improved heatconduit performance for intergrated circuit chips and equipmentutilizing such chips.

It is a further object of the invention to provide improved coolingequipment which does not cause misoperation of, or damage to, the chip.

According to one embodiment of the present invention, there is provideda cooling system comprising at least one piston having a first surfacelocated opposite a chip mounted on a substrate, a second surface forsliding the piston in a direction perpendicular to the substrate, athreaded hole in the piston for receiving a screw, and a slot portionthrough the center of axis of the screw hole which is expanded where thescrew is driven into the hole. A hat assembly can be fixed to thesubstrate and has a plurality of cavities for receiving the pistons. Thepiston location is adjusted and fixed in the cavity by expansion of thepiston due to the screws to achieve an optimum gap relative to the chipto promote efficient heat transfer without mechanical connection. Acooling fixed to the hat assembly plate has an inlet and an outlet forcirculating the cooling medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a first embodiment of the invention;

FIGS. 2A, 2B, 2C, 3 and 4 show the detailed structure of the piston ofthe FIG. 1;

FIG. 5 is a cross-sectional view of a second embodiment of theinvention;

FIGS. 6A, 6B and 7 show the detailed structure of the piston of the FIG.5;

FIG. 8 shows a cross-sectional view of apparatus for circulating thecooling medium stream; and

FIG. 9 is a cross-sectional view of a third embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Description will now be given of detailed embodiments of the invention.It will be recognized by those of skill in the art, however, thatvarious modifications may be made to the basic structures describedherein, and the description is not, therefore, intended to be limiting.

Referring to FIG. 1, a cooling system according to the present inventioncomprises a hat 17 into which pistons 15 can be inserted, frames 18 forholding a substrate 12, screws 19 for fixing the frames 18 to the hat17, and a cooling plate 20 attached to hat 17 having internal coolingpath 23 having inlet 21 and outlet 22. Chips 11 are fixed to thesubstrate 12 by solder 13, respectively. The pistons 15 are positionedso that there are small gaps 14 between the upper surfaces of the chipsand the opposing surfaces of the pistons. The most important factordetermining the cooling ability according to the present invention isthe width of the gaps 14, so that one subject matter of this inventionis to make a gap as thin as possible.

Generally speaking, since the chips 11 are fixed to the substrate 12 bysolder 13, and the range of the variation in thickness of solder 13 isnormally over 100 μm (micron), it may be considered impossible for thegaps 14 to be of less than 100 μm. On the contrary, according to thepresent invention, gaps 14 of less than 100 μm can be made in thefollowing manner.

At first, spacers (not shown in FIG. 1), for example, made from Teflonsheet of 50 μm thickness, are put on the chips 11, after the chips 11have been soldered to the substrate 12. Then the hat 17 is fixed to theframe 18 by the screws 19. Next, the pistons 15 are inserted into holesof the hat 17 and the screws 16 are driven into the holes 25 (in FIG.2A) of the pistons 15 in a manner described in detail hereinafter sothat the pistons are pressed onto the spacers on the chips 11, and theposition of the pistons in the hat is fixed.

Next, the screws 19 are removed, and the frame is disassembled from thehat so that the spacers can be removed. The frame and hat are thenreassembled with the screws 19 so that a gap space 14 corresponding tothe spacers exists between the pistons and the chips.

Next, the method whereby the pistons 15 are fixed to the hat will beexplained. Referring to FIGS. 2A, 2B, and 2C, each of the pistons 15 iscylindrical, has a threaded screw hole 25 in its center, and has a slit24 through the axis thereof. Referring to FIG. 3, the hole and screw areconfigured such that they have the same contour, but there is a gap 28therebetween.

Referring now to FIG. 4, when the screw is driven into the screw hole,the surface of the screw 16 contacts the surface of the screw hole 25,so that the pressure 29 is generated. The pressure 29 has components inthe diameter direction (30) and the axis direction (31). Since thediameter direction pressure element 30 functions to widen the slit 24 ofthe piston 15, the outer cylindrical surface of the piston 15 and theinside surface of the cavity are contacted, so that the piston 15 isfixed to the hat 17 by friction.

Referring now back to FIG. 1, after the gap 14 is set for all the chips11, the cooling plate 20 is fixed to the upper surface of the hat 17.The cooling medium is circulated through the path 23 and heat istransferred from the chips to the cooling medium across the gaps to thepistons and through the hat and cooling plate. It will be recognizedthat this system does not have the disadvantages of the prior art inthat there is no electrical or mechanical connection between the chipand the piston and the piston does not have the possibility ofvibrating.

In the embodiment described above, when the screw 16 is driven to fixthe piston 15 to the hat 17, the piston 15 may rotate. About 90% of thetorque exerted by driving the screw 16 is converted into friction power,and only the other 10% of the torque is converted into driving in theaxis direction. Accordingly, the diameter direction pressure 29 isrelatively less than the torque. As a result, the degree of fixing forpiston 15 to the surface of the hole of the hat 17 becomes relativelylow.

To solve this disadvantage, a second embodiment according to the presentinvention will be explained.

Referring to FIG. 5, a cooling system embodying to the present inventioncomprises a hat assembly 17 which has cavities 4 and guides or stops 41.The combination of the cavities 4 and the guides form and define concaveportions of the hat having a larger dimension than the cavities 4designated as 3. Frame 18 is fixed to the hat 17 by screws 19. Pistons15 adapted to receive screws 16 and having a stopper portion 42 are slidinto the holes 4. A cooling plate 20 having inlet 21 and outlet 22 isfixed to the hat assembly 17. A substrate 12 with chips 11 is fixed tothe hat assembly 17 and the frame 18 so that a small gap 14 existsbetween the surface of the chips 11 and the stopper portion 42 of thepistons 15. Referring to FIGS. 6A and 6B, it will be seen that a slit24' has been made through the center of axis of the piston 15 andextends into stopper 42. It will be seen that the stopper portion issquare and it is of a dimension such that when rotated it will engageguide 41 to prevent the piston from rotating.

Next, the method for assembling the second embodiment will be explained.Referring to FIGS. 5, 6A, 6B and 7, the hat assembly 17 is inverted.Next, one of the pistons 15 is slid into the cavity 4 and up to thestopper portion 42. The screw 16 is then driven into the threaded hole25' of the piston 15. In this manner, the piston 15 is temporarily fixedto the hat assembly. In this procedure, the rotation of the stopperportion 42 is stopped by the guide 41. The other pistons 15 aretemporarily fixed to the hat assembly 17 in the same manner. Next,spacers (not shown in the Figures) of 50 μm thickness are mounted on thestopper portions 42 of the pistons 15; then, the substrate 12 is fixedto the hat assembly 17 by the frame 18 and the screws 19 so that thechips 11 mounted on the substrate 12 face the stopper portions 42. Next,the hat assembly is turned over again. Then the screws 25 are loosened,so that the temporary fixing of the pistons 15 is undone. Then thepistons 15 are slid so that the stopper portions 42 and the spacers arein contact with the chips. Next, the screws 16 are driven again, so thatthe pistons 15 are fixed to the hat assembly 17. In this step also, therotation of the pistons 15 is prevented by abutting of the stopperportions 42 against the guides 41. After fixing the pistons to the hatassembly, the substrate 12 is removed and the spacers are removed. Thenthe substrate 12 is refixed to the hat 17. By this procedure, a smallgap 14 of desired size is formed between the chips 11 and the stopperportions 42 of the pistons. Finally, the cooling plate 20 is fixed tothe top of the hat assembly.

In either embodiment, a material 43 for reduction of friction can becoated on the screw 25 as illustrated in FIG. 7. For use as lubricatingmaterial 43, solid lubricant, for example, silicon, or molybdenumnitride powder, is better than grease or machine oil. However, grease ormachine oil can be used. The material 43 for reduction of friction iscoated on both the screw 16 and the threaded hole 25.

Referring now to FIG. 8, an embodiment of the cooling plate will bedescribed. As illustrated, the cooling path 20 is a winding one, and thepistons 15 are arranged in 6×6 matrix. The cooling path is positionedover a row or line of the pistons 15, and is then reversed, andpositioned over another row or line of the pistons 15. Since the path 20is positioned close to the pistons 15, all of the pistons are cooleduniformly and efficiently. Since the coolant is not itself in contactwith the ICs, any suitable material can be used. In particular, a liquidcoolant having a high heat capacity can be used. Suitable coolants wouldinclude water.

Referring now to FIG. 9, in this embodiment, the gap 14 between thechips 11 and the stopper portions 42 of the pistons is filled withmaterial 32 to enhance heat transfer. This material can be, for example,a silicon powder.

This present invention provides an improved cooling assembly wherein themechanical contact between the chips and the pistons is eliminated whileat the same time efficient heat transfer is achieved. According to thisinvention, the outside surfaces of the pistons 15 are closely contactedwith the inside surfaces of the holes 3 of the hat assembly 17, so thatthe performance of heat conduct can be improved, and the cooling effectof the cooling system can be improved. On the other hand, the mechanicalstrength is increased, so that the reliability of the system can beimproved.

What is claimed is:
 1. A method of producing a cooled electronicassembly comprising the steps of:(a) providing a substrate havingmounted thereon a plurality of heat generating electronic components;(b) providing a hat assembly which can be fixed to the substrate havingtherein cavities into which pistons having means for fixing theirpositions in the cavities have been slidably mounted, said cavitieslocated so as to be adjacent to said components when the substrate andhat assembly are joined; (c) placing spacers on said electroniccomponents corresponding to a desired gap distance between theelectronic components and the pistons; (d) temporarily fixing the hatassembly to the substrate; (e) slidably moving the pistons into abuttingrelationship to the spacers and fixing the pistons in their respectivecavities; (f) removing the hat assembly from the substrate and removingthe spacers; (g) reassembling the hat assembly and the substrate wherebythere is a gap between the pistons and the components; and (h) providinga cooling medium to said hat assembly.
 2. The method of claim 1 furtherincluding the step of providing a section on the pistons larger thansaid cavities adjacent to said components and providing an element onsaid hat assembly for engagement with said section to prevent rotationof said piston.
 3. The method of claim 1 wherein the means for fixingcomprises a slot in said piston into which a screw can be driven toexpand the sides of the piston into contact with the sides of thecavity.